The present invention relates to a protective device for protecting an electrical load from overheat fault in a three-phase power supply system, and more particularly to a protective circuit for preventing three-phase induction motor from overheat fault caused by undervoltage and phase failure.
Three-phase power supply system is widely used in the field of industry as a power source than single-phase power supply system for the reasons of energy efficiency, cost of device, power factor and convenience in use. However, the conventional protective device only detects any two phases of the three-phase power source. In case there are fault conditions of undervoltage and/or phase failure happen in the non-detecting phase, the protective device will not have the ability of detecting this fault. In a result, the electrical load, especially induction motor, will be damaged. Moreover, the fault condition will not be detected accurately because the actuating coil of the electromagnetic switch will have a voltage drop thereacross when the load is being powered. That is, the protective device will not cut off the load from the three-phase power source because the actuating coil of the electromagnetic switch is not sensitive to the voltage change of the power source. In a result, the fault condition will keep the electrical load powering and cause damage to the electrical load.